Dielectric wave guide to coaxial line junction



Nov. 1o, 1953 S. B. COHN DIELECTRIC WAVE GUIDE. TO COAXIAL. LINE JUNCTION Filed May 24, 1946 MI Y /AI 2 Sheets-Sheet l .SEYMOUR afcoHN ATTORNEY Nov. 10, 1953 s. B. coHN 2,659,055

DIELECTRIC WAVE GUIDE TO COAXIAL LINE JUNCTION A T TOR/VE Y Patented Nov. 10, 1953 UNITED- `sTME-s PATENT @FFI-CE DIEEECFERICS WAVE GUIDE '0 OQAXIAL JUNCTION Seymour Bf. Cohn; Cambridge, Mass., assignor to' the Unit-ed yStates of America. as' representedV by'tle'Seeretary of War Appliatmmmy sa 194s,.s .erie1lNu. 612,0174

My invention. relates to electricalV apparatus and'more particularly to `electromagnetic energy transference structures having junctions-between coaxial transmission lines. and wave guide transmission. lines.

Itis frequently desirable in the electrical? art to transfer electromagnetic energy of high or ultrahighv frequency between coaxial! lines andi wave guides. For example,A the energy generated by many ultra-high frequency osciilators is most readily transmitted therefrom by a probe meanswhich is usually extended to form the inner cone ductor ofV a. coaxial. line. A like adaptation is often utilized in coupling energy from or to res.- onant cavity structures., Coaxial transmission lines for. systems of this. nature employing firequencies in a` relatively high range,fprove` comparatively inefficient for carrying. the energy over any extended distance. Furthermore, it' is fifequently-desirabein such systems to employ Wave guide transmission lines of' various kinds to make available in the. system advantages which'V innere in certain Wave guide transmission modes: O'ne of the diilicultiesY encountered prior art junctions` between a coaxial' transmission line. and a wave guide transmission line is the limited frequency band through which the j unction may be used. Without excessive losses being introduced; These losses have occurred not only at' frequencies displaced from` thev cont'enlplatedA operating fjrequency but sometimesV even at .the `contemplated operating frequency. Various. expedients' have been proposed4 heretofore to. minimize thel undesirable eiects of a restricted frequency band" of operation andtoincrease the efifectivefrequency band ofoperationwithmore or lessl success.

It. is,A therefore, Yan'. object. of my present invention to. provide. an improved junction; between a coaxial transmission line andlawave g-iiide' transmissionline; It is afurther obfiect-.of-.myinyentionfto provide such a junction which affords an increased frequency band of .ecient operation.

It is" anotherrohject ofymyinvention-.to-fprovide ay junction' which will operate: in ai satisfactory mannerat frequencies-of operation ini. thernear neighborhood of the cutoi frequency of the-wave Itis-anotherobjection myi-invention tosprovide al Vjunction betweeni a coaxial transmission-'.- line and a. rectangularicrossesectionati wave;guide-".7a Anotherl object-tof my invention `s'rta-.lencivfdcea Ajunctionbetween.alcoairialltransniission:line and asridge-:wavt-.eguide.v L

Still another-objectief my .imnti'onsis trrprvide a junction between a coaxial transmission line andi a double ridge wave' gui'de A further objfect of' this' invention is' to" provide a high pass-lter which will have a. sharpcut-off frequency characteristic.

Still another object is to provide a waveguide section interposed in a coaxial' transmission line toV serve asa high pass filter.

Other and' further objectsy advantages.; and novel featuresv of my invention will become" ap` parent herein from' the following description talienv in` connection with tlie accompanying drawings 'it/herein: I'il` 1 is a perspective'view ofione' illustrative embodiment of' the presenti invention" showing a junction between a c'oia ial'transmissionlineand aL rectangular crossfsec nal vvaveg-i1idezV Fig. 2 is a. schematic 'e drawing'of al section taken through the line 2 -2ofFig. l.

Eig-Sis. a top-view of theembodimentfofjFig. 1`.;

Eig.v 41 is a schematic' line" drawing y'of' an illustrative embodiment' of my present invention showing', the jimction. between a coaxial transmis'- sion line and a rectangular waveguide? utilizing a. rectangular wave guide` transformer" section;

Figi 5`isl a cross-sectionalview of another illusf trative embodiment, offtle present' invej nuti'la ridge .Wave guide transformersection;

Fig. 1 is a schematic line drawinggoianother embodiment of `my invention" illustrating a" jlunction between a coaxial transmission' linie" a ridge Wave guide;r 'l

Fig. 9`is a'sclieniatic line. drawin'gof illustrative embodiment .o f the' prje's' illustrating' a jiinctionl between" .af coaxiall' miss-ion. and. a; `ridge were.' guide;

Fig. 10 isv a View .of theiembodilnentfof 9 Fig. l5 is a cross-sectional view of yet another embodiment of the invention comprising a nigh pass filter; and

Fig. 16 is a cross-sectional view of the embodiment of Fig. along the line lli-IS.

In brief, my invention contemplates a coaxial line-wave guide junction, wherein one or more sections of wave guide adjacent to and communicating with the junction have a different cross section from the wave guide to be matched.

These sections are proportional so as to improve the impedance match and the response characteristics of the junction.

Referring now more particularly to Figs. l, 2 and 3 wherein like numbers refer to like parts, there is illustrated a junction between a coaxial transmission line 20 and a waveguide 2l. Coaxial transmission line 20 has an outer conductor 22 connected to one broad wall 23 of wave guide 2i and an inner conductor 24 which extends to the other broad wall 25 of wave guide 2l and which has a portion 26 of enlarged diameter witnin the junction, as shown. A wave guide section 21 is provided which communicates directly with the junction and has its axis directly under the axis of wave guide 2! extended through the junction. Wall 23 is extended to form the top wall of section 21. It will be assumed in this description, as well as those which follow, that the coaxial transmission line extends into the top wall of the wave guide, as shown for example in the view of Fig. 2, to afford convenient reference to top and bottom Walls of the guide. Section 21 has a width a', the distance between its narrow walls, which is greater than the corresponding dimension a between the narrow walls of wave guide 2 l. Also, wave guide section 21 has a great- 4er depth b between its broad walls than the corresponding dimension b between broad walls 23 and 25 of wave guide 2l. Wave guide section 21 is snorted or closed by an end portion 28 at its far end from the junction. Consequent- 1y, section 21 may be considered as a chamber communicating with the junction.

Section 21 may have a dimension a. equal to, greater than, or less than the dimension a. I prefer to provide the end section 2l with a dimension b greater than dimension b of wave guide 2l, so that the end section has a greater characteristic impedance than that possessed by prior art conventional end sections which have the lsame inside dimensions as the wave guide to 'which the junction is made.

By increasing the characteristic impedanceof snorted section 21, its reactance may be maintained at a comparatively high value over a relatively Wide frequency range in comparison with the impedance of the wave guide to which a junction is effected. I prefer to construct section 21 to be substantially one-quarter wave guide wavelength long measured from the axis of the coaxial line termination to the closed end at about the center frequency of the operating range. I prefer to calculate the dimensions of the various parts to provide a substantially zero standing wave ratio at this frequency. In other words, the dimensions are calculated to give a perfect match at some chosen frequency. j

Referring now more particularly to Fig. 4 illustrating another preferred embodiment of my invention, there is showna junction between a coaxial transmission line 40 and a. rectangular wave guide 4I. A transformer section of wave guide 42 is inserted at the junction of the line 4 with tne wave guide structure. Wave guide secv ating range of the junction. As shown, the outer conductor of the coaxial line 40 terminates on the upper surface of the guide ll and the center conductor is extended to terminate on the lower surface thereof. I have determined that good results may be obtained by increasing the diameter of the center conductor 43 by means of a taper where it enters the wave guide to a diameter approximately 0.15 times the width of the guide. In

addition I provide a back cavity, or snorted section 44 having the same cross-sectional dimensions as main wave guide 4i. In one operative model constructed in accordance with the illustration of Fig. 4 I provide a junction between a coaxial line 40 having a characteristic impedance of 50 ohms, and a wave guide having cross-sectional dimensions of 2% inches in width by inch in height. The length of section 42 is chosen to be substantially one-quarter guide wavelength Omg/4) at frequency fo, as shown, and a terminating snorted section 44 also of length log/4 is provided, extending from the junction oppositely from wave guide 4l, as shown. Terminating secr tion i4 nas the same cross-sectional dimensions as wave guide lli. A quarter guide wavelength at a center frequency fo of 3200 megacycles was calculated to be about 1.22 inches. At this center frequency a very good match is provided by choosing the height of section 42 to be .234 inch.

Better operation near tne cutoff frequency of wave guide 4l may be obtained by reducing the cutoff frequency of transformer section 42. This may be accomplished by widening this portion of the wave guide to be greater than the 2% inches width of Wave guide 4l, or by utilizing a ridge wave guide or a double ridge wave guide for the transformer section 42.

Referring now to Figs. 5 and 6, there is illusterated a practical embodiment employing a ridge wave guide transformer section 50 to form a junction between wave guide 5i and coaxial line 52. Section 50 of the ridge wave guide has the same Width as the main wave guide 5i and a longitudinal ridge insertion 53 of length 0.92 inch, height .192 inch and width .786 inch. The ridge commences at the axis of the junction of the coaxial line, as shown in Fig. 5, which is located 1.05 inches from the snorted end of wave guide 5I. Each ofthe dimensions 0,92 and 1.05 are equal to log/4 at the operating frequency, the difference in dimensions being due to the fact that the velocity of electromagnetic energy in the ridge section is less than the velocity of electromagnetic energy in the snorted section. I nd it generally preferable in such embodiments as that of Fig. 5 to use a ridge which is between 1/3 and 2/3 of the width of the guide. The ridge section shape should be chosen so that it has a cutoif frequency about T86 of the main wave guide cutoff frequency. The main wave guide 5l has crosssectional dimensions of 2%. by 3/8 niches. Y

One advantage in employing a ridge wave guide with ridge dimensions in the region hereinbefore indicated, is that the next dominant mode of the ridge section will be equal to or greater than Athe next dominant mode of the wave guide without the ridge. Hence, if operation at frequencies near the cutoff frequency of tne next dominant mode is desired, it is advantageousthus to choose than that of the main. waveguide-i. Therev agreatr. improvement in. the; low frequency response near the cutoif frequency oil the main wane guide.-V An overall band. wdtlr of 2.400to 5300. megacycles with a standing` wave ratio-o1.' less than.v 2.0v results. OtherV useful variations: `cf this;` embodiment willll suggest; themselves,r for: example, itmayrbeadvisable to. use two y.trau'istbrmer sections which; arel substantially one-quarter wavelengths, interms erguida-wavelengths, attwo different frequencies in order .toleveli .the response characteristics.1

It may, under some circumstances, be desirable to build a junctionwhich is mismatched at the chosen intermediate frequencyiin order to afford a wider operating frequency. band wherein. the resulting standing wave ratio remains below-some predetermined gure.

-Figs.,7 to1-4 illustrate modifications ofthe emb oliment. shown in Figs. 5 and 6 which are-identicali. therewith exceptasnhereinafter set forth.

Referring now to Figs. '7l' and- 8 wherein like reierence.l numerals refer te like. partsya junction is illustrated between a coaxial transmission line and a ridge wave guide 7|. A back cavity I2 is formed on an opposite side of the junction of wave guide 1| which comprises a shorted or closed section of ridge Wave guide. Ridge 13 in section I2 extends to the junction and has different dimensions from ridge 14 which extends in the main wave guide 1|. The other internal dimensions of wave guides 1| and 12 may be the same. As before, the outer conductor 15 of coaxial line 10 terminates on the top broad wall of the wave guide junction and inner conductor 16 is extended into the junction to make contact with the ridges which extend symmetrically along the lower broad wall. The diameter of inner conductor 16 may be enlarged, as shown, where it enters the junction, to provide a more perfect impedance match. This feature may be included in the other embodiments herein disclosed.

Referring now to Figs. 9 and 10, there is illustrated a junction between a coaxial transmission line 80 and a ridge wave guide 8|. This embodiment is similar tothe embodiment illustrated by Figs. 7 and 8 except that the shorted back cavity 82 is a section of ridge wave guide wherein the rectangular ridge 83 has the same width as ridge 84 in main wave guide 8| but has a different height.

Referring now to Figs. 11 and 12 there is illustr'ated still another embodiment of the present invention showing a junction between a coaxial transmission line 90 and a ridge wave guide 9|. In this embodiment back cavity 92 is formed of a conventional section of wave guide having a smaller depth than the depth of ridge wave guide 9| with the ridge absent, but having its upper internal wall formedas an extension of the upper internal wall of wave guide 9|.

Referring now to Figs. 13 and 14, there is i1- lustrated still another illustrativeembodiment of the present invention wherein a junction is provided between a coaxial transmission line |00 and a section of double ridge wave guide |0|. The back cavity is formed of a section of double ridge wave guide |02 wherein the two symmetrical--v rectangular ridges` |1031 and HMS extendingsfmm" the` broad walls have: diiierent. dimensions from the symmetrical ridgesV |'.05f and: 1.00. of wane guide |0|;.w The ridges. |03; and (i104. of; the back cavity |302 are not. so: `deert and are` broader than thoseof main. wave guide |0,|;..

Referring now to'Figs; 153 and- IG'. whereirare illustrated two. Ycross-sectionlal viewse ofa high.- p'a-ss .filterL structure, coaxial. transmission line |20; is connected by a. junction,Y whiehmayzbei, ci the typeA previously disclosed@ herein; to; one end of a length: cf'waveA guide' i211. A sectionv junce tion, whichVv may be similar torthe.: rstjuncticn; affords; a connection between the-other' end: ct wave guide |2|- and." a.A second coaxial transmis.- sion line |22. Thejunction. includes transformer sections |2231 and`` |24i respectively composedf of shortsectionsof ridge wave guides which conr- I-nunicate directly withlthe conventional rectangular waveguide |2|`. Thejunctions-also include backA cavities or sections |25 and |226, respectively..

In the design: of this filter the-matching transfbrmersections andi cavities arel proportioned to give a match in impedance between the guiide f and the coaxial lines at frequencies well above the desired cutoff frequency'. The dimensions shown providea high-pass'lter/which will per'- mit the transfer of frequencies from one coaxial line tothe other (each having a characteristic impedance off50`-ohms) with practically no'loss above a frequency of 2700 megacycles and Vnntp about '7000i meg-acycles; Below 2700'lfmegacycles the attenuation increases with great rapidity, for example, being 14 decibels down at 2500 megacycles.

It will be apparent from the foregoing description, to those skilled in the art, that my invention is susceptible of many variations. Therefore, it is not desired to restrict the invention to the precise embodiments herein disclosed, but I desire by the accompanying claims to include all such variations as fall Within the scope and spirit of the invention.

What I claim as my invention and desire to secure by Letters Patent is:

1. An ultra-high frequency coupling device comprising: a wave guide including a rectangular rst section having a length of substantially onequarter wavelength at the operating frequency and having one end thereof closed, said i'lrst section having a given internal cross-section, and a. rectangular second section having a length of greater than one-quarter wavelength at the operating frequency, and having one end thereof joined to the other end of said -rst section, at least a portion of said second section having an internal cross-section which is appreciably less than -said given cross-section, said portion being adjacent to said first section and having at least a predetermined length, said predetermined length being substantially one-quarter wavelength at the operating frequency; a coaxial line coupled to said wave guide at the junction of said iirst and second sections, with the inner and outer conductors respectively of said line connected to the opposite wide walls of said rst and second sections.

2. A coupling device as defined in claim 1, wherein the entire second section has an internal cross-section which is appreciably less than said given cross-section, [both the wide and narrow Walls of said rst section being respectively larger than the wide and narrow walls of said second section.

3. A coupling device as defined in claim 1,

7 wherein said predetermined length is substantially one-quarter Wavelength at the operating frequency; further comprising a rectangular third section of said wave guide having a, length of substantially one-quarter Wave length at the operating frequency and having one end thereof closed, said third section having said given internal cross-section, said second section having a length which is greater than one-half Wavelength at the operating frequency and having its other end joined to the other end of said third section, said second section having a second portion having a cross-section and a length equal to said first-mentioned portion, said second portion being adjacent to said third section; and a second coaxial line coupled to said wave guide at the junction of said second and third sections, with the inner and outer conductors respective- 4ly of said second line connected to the opposite wide walls of said lsecond and third sections.

`4.. A coupling device as defined in claim 1, wherein the portion of said inner conductor Within said Wave guide has a diameter greater than one-tenth the width of the smallest wide Walls of the Wave guide. f

5. A coupling device as defined in claim 1 wherein said entire Wave guide is a single Wave guide element, and said portion is formed by a ridge extending longitudinally along the inner surface of one wide Wall of the Wave guide element.

6. A coupling device as defined in claim l wherein said portion is said predetermined length and differs in cross-section from the remainder of said second section so that it has a lower cutoff frequency than said remainder of said second section.

7. A coupling device as defined in claim 1, including a ridge extending from said inner conductor to said closed end of said first section along the inner surface of one wide wall of said first section.

8. A coupling device as dened in claim 7, wherein said portion includes a ridge extending longitudinally throughout said portion along the inner surface of one wide Wall of said portion.

9. A coupling device as dened in claim 1, including a ridge extending longitudinally along the inner surface of each Wide Wall of said portion throughout said portion.

SEYMOUR B. COHN.

References Cited in the file of this patentV UNITED STATES PATENTS Number Name Date 2,232,179 King Feb. 18, 1941 2,404,086 `Okress July 16, 1946 2,408,032 Beck Sept. 24, 1946 2,431,941 Kihn Dec. 2, 1947 2,476,732 Hollingsworth July 19, 1949 2,526,678 Mallett Oct. 24, 1950 2,531,437 Johnson Nov. 28, 1950 

